The present disclosure relates in general to fabrication of micro-electro mechanical systems (MEMS) devices, and more particularly, to wafer backside alignment in fabrication of MEMS devices. The present disclosure also relates to an improved method and system for wafer backside alignment in fabrication of MEMS devices.
MEMS devices are common in applications, including wafer leveling packaging, integrated optics, pressure sensors, compound devices, and backside vias. In fabrication of three-dimensional devices such as MEMS, the substrate is processed on one side, flipped over, and is processed on the opposite side to create a desired three-dimensional structure. Front side and backside alignment are performed to ensure that the three-dimensional structure is properly aligned. For example, if a contact runs through the substrate from the front side to the backside, it must be precisely aligned to other elements of the device, such that electrical contacts can be made.
Currently, a number of methods are used to align the backside substrate from the front side, including blind stepping, double-side aligner, and backside alignment with embedded optics. Blind stepping is only suitable for low end products, and does not provide alignment of successive patterns on the front side. In addition, the reliability of blind stepping is low. Double-side aligner involves automatic alignment of the front side by detecting the front side alignment mark, but the wafer backside position is captured manually with a microscope. Thus, double-side aligner is a semi-automatic process that requires manual handling and software adjustment. The reliability of the process also depends on the handling skill. In addition, the overlay shift of a double-side aligner is around 2 μm, which indicates a low overlay accuracy.
Backside alignment with embedded optics involves the use of prisms to reflect the front side alignment mark when the wafer backside faces up. This requires addition of embedded optics by the step and scan tool. However, this method is complex in that the alignment laser must pass through a complex route. In addition, the signal strength of the alignment light is a weak point of the alignment system design. Furthermore, the overlay shift of backside alignment with embedded optics is around 0.18 μm.
Therefore, a need exists for an improved method and system for wafer backside alignment that is simple and robust, provides a quick measurement of various alignment sites on a substrate such that it can be quickly aligned and processed, and provides a satisfactory overlay accuracy.